Wednesday, January 23, 2013

Jan 23, 2103: Light Snow Tonight, Light Snow Friday


Mother Nature is trying very hard to make it snow this winter.   We finally have arctic-cold air in place, but we are lacking moisture and strong storms.   This trend will continue through the end of the week.

The first shot of light snow comes tonight.   As shown in the figure below, the computer models suggest an Arctic Clipper system will race across the Mid Atlantic tonight.  Arctic Clippers are weak upper air disturbances, moving along a fast jet stream oriented roughly west to east.   They are often associated with a blast of arctic air.   The figure shows the track of the Clipper system, across the Ohio Valley and Mid Atlantic.  The magenta colors portray an area of very light snow.  The surface low pressure associated with this Clipper is located over southwest Virginia at 1 AM.  Note the extremely intense high pressure region (1045 mb) over the Upper Midwest:  This is the reinforcing blast of arctic air on the heels of the Clipper.  The core of the really chilly stuff will stay just north of our region.  But 1045 mb is a very, very strong high pressure not often seen in any given year.

Forecast surface chart for 1 AM Thursday
Clippers are marginal snow-producers, because (1) they are moisture-starved (there is no inflow of air off the Atlantic or Gulf of Mexico) and (2) fast-moving (little time for snow to accumulate).  Furthermore, their snow:liquid ratio is on the order of 15:1 to 20:1, because the air is so dry and cold (our big coastal snow-maker storms have ratios on the order of 7:1 to 8:1, giving a heavy, wet snow).   Snow grains produced by Clippers are tiny and fluffy, leaving a fine powder on the surface.  It only takes b/t 0.05-0.1 inch of liquid equivalent to produce one inch of snow in a Clipper.  I find an effective way to clear our cars and driveway after one of these systems is to use a leaf blower, not a shovel.  With our roads now frozen, even an inch of this powder could create problems for the Thursday morning rush-hour.

Snow accumulation map issued this afternoon by NWS Sterling, VA:

Snow accumulation for early Thursday AM. NWS.

The second shot of light snow arrives Friday, in time for the Friday evening rush hour.  Again, amounts are expected to be light.  This is because there are two pieces of energy moving through the Mid Atlantic, and the models suggest that these pieces will not combine or phase in time to create an intense storm:

Forecast surface chart for 1 PM Friday

The phasing will take place out over the Atlantic.  The air mass in place will be cold enough for all snow, and once again we will be dealing with a light, powdery type of snow.  If anything falls, it will come from the northern ripple, which is another Alberta Clipper (heralding in yet another shot of arctic air for the weekend!).  It is even possible that our region will escape snow altogether - essentially a "snow hole" in between these two systems.  But should a light coating fall, again...the rush hour could be in peril.

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